The most direct two senses of human beings are vision and audition, and thus scientists have been endeavored to develop various sight and sound reproduction systems for a long time. Currently, a moving-coil loudspeaker plays a dominant role in the entire loudspeaker market. In the recent years, however, to bring more sonic sensuality and comply with requirements for short, tiny, small, and compact 3C (computer, communication, and consumer electronics) products, a power-saving compact speaker with a proper ergonomic design is going to be far more extensively applied in various forms, such as a large-size flat loudspeaker, an earphone set of a walkman, a cellular phone with three-dimensional surrounding sound effects, and so on.
At present, a loudspeaker can be categorized into a direct-radiating speaker and an indirect-radiating speaker. Besides, based on a driving mechanism, the loudspeaker can be classified into a moving-coil loudspeaker, a piezoelectric speaker, and an electrostatic loudspeaker. The moving-coil loudspeaker is the most common and mature speaker by now, while the intrinsic structural properties of the moving-coil loudspeaker do not conform to the requirements for miniaturizing the 3C products and reducing the size of home theater systems.
The piezoelectric speaker uses piezoelectric materials that have the property of converting electrical energy into mechanical energy by undergoing a controllable amount of deformation when subjected to an applied electric field. Thereby, vibrating membranes in the piezoelectric speaker can make a sound. Note that the piezoelectric speaker has a compact size. The electrostatic loudspeaker is now mainly applied to hi-end headsets and audio systems. In a conventional electrostatic loudspeaker, a capacitor is formed by a conductive vibrating membrane sandwiched between two perforated fixed electrode boards. By supplying a direct bias voltage to the conductive vibrating membrane and supplying an alternating voltage to the two fixed electrode boards, the conductive vibrating membrane oscillates because of the electrostatic force generated by positive and negative electric fields, such that sound is radiated. The direct bias voltage provided to the conventional electrostatic speaker must reach hundreds or even thousands of volts, and therefore an amplifier with high unit price and significant volume is required. Said disadvantage discourages popularization of the conventional electrostatic speaker.
When the electrostatic speaker requiring high voltages is operated, electromagnetic interference (EMI) can be expected. Hence, to be compliant with relevant international standards, the disclosure is directed to a flat loudspeaker structure capable of preventing the EMI. With proper driving modules, the utmost electrodes of the flat loudspeaker structure are grounded to not only prevent the EMI but also protect a user from a risk of electrocution. Besides, when a sound pressure power level is increased, an issue arising from the complicated structure and circuits of the conventional flat loudspeaker structure can also be resolved according to the disclosure. With the simple structure, the flat loudspeaker of the disclosure can be mass-produced by performing existing manufacturing processes.
In the future, audio plays an important role in applications of soft electronics. Since the soft electronics are soft, thin, low-power driven, and flexible, how to achieve the breakthrough of the conventional design and fabricate the parts equipped with soft electronic properties remains as one of the main purposes of the disclosure.